This invention relates to tightening methods for threaded fasteners and in particular to a tightening method for reducing the variation in fastener tensions resulting from relaxation.
The use of threaded fasteners to connect materials together is well known. As used herein, threaded fasteners include: bolts and nuts, bolts received in tapped holes, studs with nuts, and the like. The fasteners and the elements that are fastened together are collectively termed a joint.
The compression of the joint and the tension along the shank of the threaded fastener affect the quality of the joint. In joints subject to shear, higher compression forces increase the shear resisting friction between the joint elements. Hence, increased fastener tension may be preferred as limited by the proof strength of the fastener. In joints subject to axial load, i.e., directed along the shank of the fastener, fastener tension must be limited so that the yield strength of the fastener is not exceeded under peak loads. Fluctuating axial loads, which may induce fatigue in the fastener, place different limits on the optimal tension in the fastener.
Fastener tension is difficult to measure directly. Instrumented bolts, load washers and certain ultrasonic techniques have all been used to provide essentially direct indications of fastener tension, however, these techniques are expensive or time consuming and generally not feasible in production environments. Fastener tension is instead, more typically deduced from measurements of fastener torque, such as in "torque control methods" or measurements of the angle of rotation of the fastener, such as in "turn-of-the-nut methods". Both fastener torque and angle of rotation can be easily observed during the assembly of the joint.
The relationship between fastener torque and tension is highly dependant on the coefficient of friction between the fastener and the elements of the joint to be fastened. This coefficient of friction varies considerably between joints for reasons including variations in surface coatings or lubrication of the fastener as well as imperfections in the fastener threads.
The relationship between angle of rotation of the fastener and the fastener tension is correspondingly dependant on the relative stiffness of the joint elements and the fastener, quantities that also vary considerably between joints.
The effects on these variations in joint friction and stiffness may be reduced by sophisticated combinations of torque and angle measurement. For example, the logarithmic rate method ("LRM") measures the change of torque with angle during the tightening of the fastener and thereby estimates the effects of friction and joint stiffness. The LRM method is described in a paper by S. Eshghy entitled: "The LRM Fastening System" (AD77-716), published by the Society of Manufacturing Engineers, Dearborn, Michigan. U.S. Pat. No. 4,245,381 entitled: "Tension Control of Fasteners", issued Jan. 20, 1981, and incorporated by reference, also provides a detailed description of LRM fastening system as well as descriptions of the above mentioned torque control and turn-of-the-nut fastener tightening methods.
In situations where a joint is assembled with multiple fasteners, uniformity of fastener tension is also critical. Uniform fastener tension prevents distortion of the joint elements and ensures that each fastener efficiently shares the load placed on the joint. Sophisticated fastener tightening schedules, such as LRM, improve uniformity of fastener tension by increasing the accuracy of fastener tension under variations in joint friction and stiffness.
It is known in the art that fasteners that have been assembled under tension in a joint will experience a tension loss over time. This tension loss is termed "relaxation" and is manifested in a corresponding loss of torque which may be detected by a procedure referred to as a "static audit". In a static audit, torque is re-applied to a previously tightened fastener some time after the fastener was initially tightened. The maximum torque measured during the audit is considered to be the "static torque". As used herein, static torque is defined as the resistive torque exhibited by a fastener turning at an arbitrarily low reference speed. The maximum torque observed just before the fastener resumes motion is termed the "breakaway torque". As a result of varying degrees of relaxation, therefore, the static torque measured in the static audit will frequently vary considerably among identically tightened fasteners.
Tension relaxation appears to have two components which may be termed: "quick relaxation" and "time relaxation". Quick relaxation occurs very shortly after the applied torque is removed from the fastener 36 and is thought to result from a slight unwinding of the fastener 36 produced from relaxation of torsional stress in the fastener shank. Second order elasticity terms in the elasticity of the fastener shank, as opposed to the first order term of Hook's law, may also produce this loss of tension with loss of torsion.
Time relaxation occurs over a longer period of time and is believed to be caused by plastic flow or creep of the joint 34 materials themselves, including gasket 42 , and may occur over the span of several minutes to several days.
Various methods have been proposed to reduce relaxation of fastener tension. In one approach a pulsed application of a recovery torque equal to the dynamic torque of the fastener, follows immediately after the initial fastener tightening. Dynamic torque, as used herein, will be defined as the final torque achieved in the initial tightening of the fastener yet while the fastener is still turning at a tightening speed which is substantially higher than the previously mentioned reference speed of the static audit.
The above described pulsed application of a recovery torque equal to the dynamic torque is not effective in situations where the dynamic torque is substantially below the breakaway torque of the fastener, the latter which may remain high even as the joint tension relaxes. In such instances, the tension of the fastener is unaffected by the pulsed recovery torque.
A static audit is ordinarily conducted with a hand-held torque wrench. The torque on the fastener is slowly increased until the fastener breaks free. The maximum torque on the wrench is then recorded. Measurements of static torque will vary depending on the speed with which the fastener is turned and the distance it travels, neither of which may be easily controlled by hand. Joints that break violently, or very hard joints requiring large torques (200 Nm or greater) can be difficult to audit by hand. Operator fatigue may lead to errors. Also the torque wrench only records only a single peak torque, so it cannot be determined whether the breakaway torque or the torque rise due to angular advance was measured.